Soil transmitted parasitic nematodes are world wide one of the most commonly acquired infections with multi-cellular parasites. The adaptive host response in humans and mice is characterized by the presence of CD4+ Th2 cells and their signature cytokine IL-4. Th2 cells and IL-4 are associated with protective immune responses against helminth parasites they are detrimental in certain immunopathologies such as antigen-induced asthmatic reactions, atopy and allergy. While Th2 cells are associated with nematode infections; however, it is not clear how Th2 cells develop from naive CD4+ T cells in response to infection. Numerous studies have shown that IL-4Ra-mediated signals and IL-4 are required for the Th2 development of naive CD4+ T cells in vitro, however, very little is known about these processes in vivo. This is largely due to the difficulty to identify and track Th2 cells defined by IL-4 expression. To overcome these limitations we have developed bicistronic IL-4 reporter (4get) mice. In contrast to in vitro studies, our preliminary data show that Th2 priming occurs surprisingly efficient in IL4Ra-/- 4get mice infected with the murine helminth H. polygyrus. Thus, the role of IL-4R and IL-4 for Th2 development is controversial. Recently we have generated novel IL-4 dual-reporter mice (4get/KN2) and revealed that IL-4 competence and IL-4 production are distinct steps. Here we propose a novel model whereby Th2 development and IL-4 production occur in several distinct, identifiable steps: 1. activation, 2. differentiation, 3. expansion, 4. IL-4 production, 5. dissemination. In the current application we will revisit the role of IL-4 and IL-4Ra-mediated signals for Th2 differentiation and IL-4 production using our unique dual-reporter mouse model. We hypothesize that IL-4R functions are not required to nucleate the Th2 priming of CD4+ T cells but play a role in multiple other steps of this model. In Aim 1 we will analyze which step(s) in Th2 differentiation of the endogenous T cell population are regulated by IL-4R signals in response to infection. In Aim 2 we will determine which step(s) in Th2 differentiation are regulated by IL-4R signals mediated directly on naive, antigen-specific CD4+ T cells. We will adoptively transfer apTCR transgenic CD4+ T cells and immunize with the cognate antigen using H. polygyrus larvae as a Th2 adjuvant. In Aim 3 we will dissect the role of IL-4 and IL-4Ra expression by selective lineages in the multi-step process of Th2 differentiation. We will generate various bone marrow chimeras which lack the respective components in selective cellular lineages and follow the development of the endogenous T helper response to infection. Collectively the proposed Aims will reveal the mechanism by which IL-4R and IL-4 regulate the multiple steps of Th2 development in vivo. These insights are valuable for designing agonistic and antagonistic intervention strategies targeting the IL-4R and IL-4. [unreadable] [unreadable] [unreadable]